Conventionally, for secondary batteries which are able to be charged and require constant-voltage charging, lithium ion batteries are developed. This lithium ion battery is charged with the characteristics shown, for example, in FIG. 1. FIG. 1 is a characteristic diagram of charging current/voltage vs. elapsed time of a general lithium ion battery, in which charging is carried out with a charging current I set as a constant current from the initiation of charging until the battery voltage reaches a specified potential. Carrying out this constant-current charging increases a battery voltage V and when it exceeds a specified value, charging is changed over to constant-voltage charging. In this event, for example, voltage V.sub.1 corresponding to battery voltage when the lithium ion battery is fully charged (that is, 100% charged) is supplied. Carrying out this constant-voltage charging charges the lithium ion battery, causes the battery voltage to rise to voltage V.sub.1, but as this charging takes place, the charging current I decreases. Now, when this charging current I decreases to a specified value, it is judged that the lithium ion battery is 100% charged (or charged nearly to 100%), and supply of charging current is stopped.
Charging in this way allows the lithium ion battery to be efficiently charged to 100%.
Now, the lithium ion battery charged to 100% in this way may sometimes have the characteristics deteriorated by the charging condition thereafter. That is, if the voltage V.sub.1 corresponding to battery voltage when the lithium ion battery is fully charged is constantly applied to the 100% charged lithium ion battery from the charging equipment as charging voltage and small-power charging is repeatedly carried out, the charging condition can be maintained to nearly 100% condition even when there is self-discharge. However, when such nearly 100% condition continues, the lithium ion battery becomes characteristics which tends to gradually reduce the chargeable capacity, and eventually deteriorates the characteristics as a secondary battery.
In order to prevent characteristics deterioration due to the continuation of the 100% charged condition, for example, stopping charging at about 90% of the charging capacity is assumed, but this results in inconvenience that the capacity prepared as a secondary battery is not effectively utilized.
When temperature of the battery itself rises, the lithium ion battery has a disadvantage that the chargeable capacity decreases and battery characteristics rapidly deteriorate, and it also has a disadvantage that it is not preferable to be charged to the full charging level with the battery temperature increased at the time of charging under the same conditions as those free of temperature rise.
As described above, it is when there remains scarcely charged voltage in the lithium ion battery to carry out constant-current charging at first and then change over to constant-voltage charging to charge the battery, and when any voltage remains in the battery, it is necessary to carry out constant-voltage charging with the charging current reduced, thereby preventing deterioration of characteristics as a secondary battery resulting from rapid charging by large current.
Consequently, before charging is started, the condition of the battery to be charged must be detected and the remaining voltage must be detected. In order to detect this remaining voltage, charging is carried out with a small current called pre-charging at the start of charging, the battery voltage, etc. at that time is detected, and the remaining voltage of the battery is detected.
FIG. 2 shows one example of a circuit configuration of a conventional charging equipment which can carry out the pre-charging, in which to one end on the secondary side (primary side is omitted) of a switching transformer 1 that composes the switching power supply, the anode of diode 2 is connected, and the cathode of this diode 2 and the other end on the secondary side of a transformer 1 are connected with a capacitor 3, and direct current power supply of a specified voltage is obtained by rectification by diode 2 and smoothing by capacitor 3.
The cathode of diode 2 is connected to one end (positive electrode) of a secondary battery (lithium ion battery) 4 loaded to this charging equipment via an opening and closing switch SW1, and the other end (negative electrode) of this secondary battery 4 is connected to the other end on the secondary side of the switching transformer 1. In parallel to the opening and closing switch SW1, a series circuit comprising an opening and closing switch SW2 and a resistor 5 is connected.
In this event, opening and closing of switches SW1 and SW2 are controlled by a control circuit 6. This control circuit 6 is connected in such a manner that the power supply is fed from the secondary side of the switching transformer 1 and is operated by this power supply. And this control circuit is designed to detect the condition of the secondary battery 4 by some method not illustrated (for example, detection of battery voltage).
To explain the control by the control circuit 6, at the start of charging, the switch SW2 is held closed, while the switch SW1 is held open. Keeping the switches in this condition allows the power supply to be fed from the secondary side of switching transformer 1 with the resistor 5 connected to the secondary battery 4 in series, reduces the charging current to be fed to the secondary battery 4 as much as the loss caused by this resistor 5, and allows pre-charging by small current to take place. And under this pre-charging condition, the battery condition such as battery voltage of the secondary battery 4 or the like is detected by the control circuit 6, and if the detected condition is judged to be the condition with little remaining voltage that allows rapid charging, switch SW1 is held closed, switch SW2 is held open, charging current is supplied to the secondary battery 4 with the resistor 5 in the condition free of loss, and rapid charging by large current is begun.
Now, FIG. 3 shows the charging characteristics when switch SW1 is turned on and those when switch SW2 is turned on, indicating that comparatively large current is allowed to flow at a given voltage as charging characteristics when switch SW1 is turned on. The charging characteristics when switch SW2 is turned on are such that the current value is suppressed to a small value.
In this way, configuring to provide a plurality of paths for accommodating the charging current and to vary the charging current to carry out pre-charging results in complicated configuration of the charging equipment as much and constitutes an inconvenience.
As a configuration of another charging equipment for enabling conventional pre-charging, there is one with the circuit configuration shown in FIG. 4. In the case of this circuit, the cathode of diode 2 is connected to one end (positive electrode) of the secondary battery 4 (lithium ion battery) mounted to this charging equipment via an opening/closing closing switch SW3, and the other end (negative electrode) of this secondary battery 4 is connected to the other end on the secondary side of the switching transformer 1.
And opening and closing of the switch SW3 are controlled by a control circuit 7. This control circuit 7 is connected in such a manner that the power supply is fed from the secondary side of the switching transformer 1. And this control circuit is designed to detect the condition of the secondary battery 4 by some method not illustrated (for example, detection of battery voltage), and opening and closing of switch SW3 are controlled based on the detected condition.
Now, to explain the control condition of switch SW3, in carrying out ordinary charging (rapid charging, etc.), switch SW3 is continuously held closed, and in carrying out pre-charging with small current, opening and closing of switch SW3 are repeatedly carried out. That is, for example, as shown in FIG. 5, when precharging is carried out at the start of charging, ON/OFF of switch SW3 are repeated to intermittently supply a specified current value I to the secondary battery 4, and average charging current is lowered, bringing about the conditions in which pre-charging Pre is able to be carried out by small current. When precharging is switched to ordinary charging, switch SW3 is continuously held closed, and charging by the specified current value I is continuously carried out.
Pre-charging by intermittently opening and closing this switch in this way enables both ordinary charging and pre-charging only by installing one switch, but at the time of pre-charging by ON/OFF of this switch, the peak current when opening and closing of the switch are changed over is transmitted to the control circuit 7, and there is a high possibility to adversely affect operation of the control circuit 7. Consequently, pre-charging by repeating ON/OFF of the switch in this way is not preferable.
Another problem is an error in detection of battery voltage in the battery charger, and there is a case in which the error deteriorates the battery characteristics.
That is, FIG. 6 shows one example of the charging control condition when the conventional lithium ion battery is 100% charged. For example, suppose that a lithium ion battery with voltage V.sub.1 when fully charged is charged and the battery voltage of this battery reaches V.sub.1 at a certain timing t.sub.1. In this event, this lithium ion battery is judged to be fully charged and supply of charging current is stopped. Stopping charging causes the lithium ion battery to gradually reduce battery voltage due to self discharge or discharge to the load circuit.
Now, the charging circuit is set to restart charging when the battery voltage reaches a predetermined voltage V.sub.2. Suppose that the charging circuit detects this battery voltage V.sub.2 at timing t.sub.2, then, charging is restarted at this timing t.sub.2, and the battery voltage rises again as shown with characteristic Vx, achieving the fully charged condition.
By setting in this way, the battery voltage is able to be held to the voltage close to full charging. Now the lithium ion battery is constantly charged nearly 100% by bringing the voltage V.sub.2 to restart charging to a voltage value extremely close to the battery voltage V.sub.1 when fully charged, but since achieving this state accelerates deterioration of the battery, voltage V.sub.2 to restart charging shall be set to the voltage slightly reduced from the battery voltage V.sub.1 when fully charged to allow the remaining battery voltage to vary in a certain range, thereby preventing deterioration of the battery.
However, in general, in the voltage detection circuit with comparatively simple configuration built into this kind of battery charger, it is difficult to constantly accurately detect voltage V.sub.12, and an error .DELTA.V is generated in the detection value of the voltage. Now, as shown in FIG. 6, if a voltage higher than actual setting by the error .DELTA.V is judged to be the voltage V.sub.1 (timing t.sub.2 '), the battery returns to the fully charged condition more quickly than the originally set condition (condition by characteristic Vx) as in the case of the characteristic Vy shown with broken line, and deterioration of the battery characteristics is accelerated.
In addition, when the secondary battery is charged to full charge and charging is stopped, depending on the condition on the battery charger side, there is a case in which discharge current is generated from the secondary battery to the battery charger side, and in such event, charging of the secondary battery is restarted in a short time, shortening the frequency to carry out charging.